The lubrication system of an internal combustion engine circulates oil under pressure through the engine block and the cylinder head for lubricating and cooling its various moving parts, such as camshaft, crankshaft, pistons, and other bearings. Different variants of lubrication system have been proposed, which have had the function of varying the oil feed of the lubricating oil pump in relation to the requirements of the engine in order to reduce power losses. Use is usually made of a pressure-reducing valve on the delivery side of the pump, which valve makes it possible to return a surplus of oil to the suction side of the pump when pressure and flow to the consumers of the engine are sufficient.
One disadvantage of these known lubrication systems is that they do not take account of changes in the viscosity of the lubricating oil as a consequence of temperature variations. Moreover, the system pressure is affected by changes in pressure drop, for example owing to an oil cooler and oil filter being partly blocked by impurities. In this connection, the pressure adjustment described above can result in some consumers not receiving sufficient oil pressure for lubrication or cooling.